netfilter: bpf: defer hook memory release until rcu readers are done

netfilter: bpf: defer hook memory release until rcu readers are done

Yiming Qian reports UaF when concurrent process is dumping hooks via
nfnetlink_hooks:

BUG: KASAN: slab-use-after-free in nfnl_hook_dump_one.isra.0+0xe71/0x10f0
Read of size 8 at addr ffff888003edbf88 by task poc/79
Call Trace:
 <TASK>
 nfnl_hook_dump_one.isra.0+0xe71/0x10f0
 netlink_dump+0x554/0x12b0
 nfnl_hook_get+0x176/0x230
 [..]

Defer release until after concurrent readers have completed.

Reported-by: Yiming Qian <yimingqian591@gmail.com>
Fixes: 84601d6ee68a ("bpf: add bpf_link support for BPF_NETFILTER programs")
Signed-off-by: Florian Westphal <fw@strlen.de>